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    /(^a|b)c/;   # matches 'ac' at start of string or 'bc' anywhere

    /house(cat|)/;  # matches either 'housecat' or 'house'
    /house(cat(s|)|)/;  # matches either 'housecats' or 'housecat' or
                        # 'house'.  Note groups can be nested.

    "20" =~ /(19|20|)\d\d/;  # matches the null alternative '()\d\d',
                             # because '20\d\d' can't match

=head2 Extracting matches

The grouping metacharacters C<()> also allow the extraction of the
parts of a string that matched.  For each grouping, the part that
matched inside goes into the special variables C<$1>, C<$2>, etc.
They can be used just as ordinary variables:

    # extract hours, minutes, seconds
    $time =~ /(\d\d):(\d\d):(\d\d)/;  # match hh:mm:ss format
    $hours = $1;
    $minutes = $2;
    $seconds = $3;

In list context, a match C</regex/> with groupings will return the
list of matched values C<($1,$2,...)>.  So we could rewrite it as

    ($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/);

If the groupings in a regex are nested, C<$1> gets the group with the
leftmost opening parenthesis, C<$2> the next opening parenthesis,
etc.  For example, here is a complex regex and the matching variables
indicated below it:

    /(ab(cd|ef)((gi)|j))/;
     1  2      34

Associated with the matching variables C<$1>, C<$2>, ... are
the B<backreferences> C<\1>, C<\2>, ...  Backreferences are
matching variables that can be used I<inside> a regex:

    /(\w\w\w)\s\1/; # find sequences like 'the the' in string

C<$1>, C<$2>, ... should only be used outside of a regex, and C<\1>,
C<\2>, ... only inside a regex.

=head2 Matching repetitions

The B<quantifier> metacharacters C<?>, C<*>, C<+>, and C<{}> allow us
to determine the number of repeats of a portion of a regex we
consider to be a match.  Quantifiers are put immediately after the
character, character class, or grouping that we want to specify.  They
have the following meanings:

=over 4

=item *

C<a?> = match 'a' 1 or 0 times

=item *

C<a*> = match 'a' 0 or more times, i.e., any number of times

=item *

C<a+> = match 'a' 1 or more times, i.e., at least once

=item *

C<a{n,m}> = match at least C<n> times, but not more than C<m>
times.

=item *

C<a{n,}> = match at least C<n> or more times

=item *

C<a{n}> = match exactly C<n> times

=back

Here are some examples:

    /[a-z]+\s+\d*/;  # match a lowercase word, at least some space, and
                     # any number of digits
    /(\w+)\s+\1/;    # match doubled words of arbitrary length
    $year =~ /\d{2,4}/;  # make sure year is at least 2 but not more
                         # than 4 digits
    $year =~ /\d{4}|\d{2}/;    # better match; throw out 3 digit dates

These quantifiers will try to match as much of the string as possible,
while still allowing the regex to match.  So we have

    $x = 'the cat in the hat';
    $x =~ /^(.*)(at)(.*)$/; # matches,
                            # $1 = 'the cat in the h'
                            # $2 = 'at'
                            # $3 = ''   (0 matches)

The first quantifier C<.*> grabs as much of the string as possible
while still having the regex match. The second quantifier C<.*> has
no string left to it, so it matches 0 times.

=head2 More matching

There are a few more things you might want to know about matching
operators.  In the code

    $pattern = 'Seuss';
    while (<>) {
        print if /$pattern/;
    }

perl has to re-evaluate C<$pattern> each time through the loop.  If
C<$pattern> won't be changing, use the C<//o> modifier, to only
perform variable substitutions once.  If you don't want any
substitutions at all, use the special delimiter C<m''>:

    $pattern = 'Seuss';
    m'$pattern'; # matches '$pattern', not 'Seuss'

The global modifier C<//g> allows the matching operator to match
within a string as many times as possible.  In scalar context,
successive matches against a string will have C<//g> jump from match
to match, keeping track of position in the string as it goes along.
You can get or set the position with the C<pos()> function.
For example,

    $x = "cat dog house"; # 3 words
    while ($x =~ /(\w+)/g) {
        print "Word is $1, ends at position ", pos $x, "\n";
    }

prints

    Word is cat, ends at position 3
    Word is dog, ends at position 7
    Word is house, ends at position 13

A failed match or changing the target string resets the position.  If
you don't want the position reset after failure to match, add the
C<//c>, as in C</regex/gc>.

In list context, C<//g> returns a list of matched groupings, or if
there are no groupings, a list of matches to the whole regex.  So

    @words = ($x =~ /(\w+)/g);  # matches,
                                # $word[0] = 'cat'
                                # $word[1] = 'dog'
                                # $word[2] = 'house'

=head2 Search and replace

Search and replace is performed using C<s/regex/replacement/modifiers>.
The C<replacement> is a Perl double quoted string that replaces in the
string whatever is matched with the C<regex>.  The operator C<=~> is
also used here to associate a string with C<s///>.  If matching
against C<$_>, the S<C<$_ =~> > can be dropped.  If there is a match,
C<s///> returns the number of substitutions made, otherwise it returns
false.  Here are a few examples:

    $x = "Time to feed the cat!";
    $x =~ s/cat/hacker/;   # $x contains "Time to feed the hacker!"
    $y = "'quoted words'";
    $y =~ s/^'(.*)'$/$1/;  # strip single quotes,
                           # $y contains "quoted words"

With the C<s///> operator, the matched variables C<$1>, C<$2>, etc.
are immediately available for use in the replacement expression. With
the global modifier, C<s///g> will search and replace all occurrences
of the regex in the string:

    $x = "I batted 4 for 4";
    $x =~ s/4/four/;   # $x contains "I batted four for 4"
    $x = "I batted 4 for 4";
    $x =~ s/4/four/g;  # $x contains "I batted four for four"

The evaluation modifier C<s///e> wraps an C<eval{...}> around the
replacement string and the evaluated result is substituted for the
matched substring.  Some examples:

    # reverse all the words in a string
    $x = "the cat in the hat";
    $x =~ s/(\w+)/reverse $1/ge;   # $x contains "eht tac ni eht tah"

    # convert percentage to decimal
    $x = "A 39% hit rate";
    $x =~ s!(\d+)%!$1/100!e;       # $x contains "A 0.39 hit rate"

The last example shows that C<s///> can use other delimiters, such as
C<s!!!> and C<s{}{}>, and even C<s{}//>.  If single quotes are used
C<s'''>, then the regex and replacement are treated as single quoted
strings.

=head2 The split operator

C<split /regex/, string> splits C<string> into a list of substrings
and returns that list.  The regex determines the character sequence
that C<string> is split with respect to.  For example, to split a
string into words, use

    $x = "Calvin and Hobbes";
    @word = split /\s+/, $x;  # $word[0] = 'Calvin'
                              # $word[1] = 'and'
                              # $word[2] = 'Hobbes'

To extract a comma-delimited list of numbers, use

    $x = "1.618,2.718,   3.142";
    @const = split /,\s*/, $x;  # $const[0] = '1.618'
                                # $const[1] = '2.718'
                                # $const[2] = '3.142'

If the empty regex C<//> is used, the string is split into individual
characters.  If the regex has groupings, then list produced contains
the matched substrings from the groupings as well:

    $x = "/usr/bin";
    @parts = split m!(/)!, $x;  # $parts[0] = ''
                                # $parts[1] = '/'
                                # $parts[2] = 'usr'
                                # $parts[3] = '/'
                                # $parts[4] = 'bin'

Since the first character of $x matched the regex, C<split> prepended
an empty initial element to the list.

=head1 BUGS

None.

=head1 SEE ALSO

This is just a quick start guide.  For a more in-depth tutorial on
regexes, see L<perlretut> and for the reference page, see L<perlre>.

=head1 AUTHOR AND COPYRIGHT

Copyright (c) 2000 Mark Kvale
All rights reserved.

This document may be distributed under the same terms as Perl itself.

=head2 Acknowledgments

The author would like to thank Mark-Jason Dominus, Tom Christiansen,
Ilya Zakharevich, Brad Hughes, and Mike Giroux for all their helpful
comments.

=cut